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Abstracts Resulting from 2009 Workshop

Home and Away: Are introduced plant species really special? (Abstract Author: Jennifer Firn)

It is often noted that plant species are more abundant in their introduced than in their native ranges. A number of hypotheses have been proposed to explain this apparent advantage, including release from enemies, competitors, and parasites, but few studies have systematically addressed the generality of this observation. Quantitative assessments of species abundance in native and introduced ranges generally focus on species known to have the highest impacts. This biased sampling may lead to overestimates of the advantage obtained when a species is introduced into a new range.

To examine this possible bias, we evaluated plant composition data collected from AB grassland communities on XY continents for plant species present at sites in both their native and introduced range. Across all sites, there were 19 (or more) species that occurred in both native and introduced locations. We considered a species dominant if it was found in all plots surveyed at a site.

We determined the patterns of species relative abundance for all 19 species and found that none were subordinate in both their native and introduced range. We found 3 out of the 19 species were dominant in their introduced range and subordinate in the native range. Half of the remaining species were dominant in both their introduced and native ranges. While the other half were subordinate in their introduced range and dominant in their native range (suggesting a potential to increase in abundance).

We also investigated if there were any attributes associated with the species or the assemblages in which they were found that enable us to predict a plant species' behaviour home and away. We compared the functional traits (life-form, life-span and resource acquisition) of all species present at the paired native and introduced sites, and assessed whether the introduced species added, filled or randomly contributed to species richness. Finally, we compared functional traits of resident species and abiotic characteristics across all sites within the global network seeking a set of attributes that distinguish assemblages dominated by introduced and native species.

Global trends of major biome vegetation structure and function across latitudinal and mean precipitation gradients have been characterized in the literature. However, controls on these vegetation characteristics within a biome are not well understood. Grasslands, for example, persist in diverse biogeographic regions throughout the world and are subject to diverse abiotic conditions that are likely to control biodiversity and primary production within and among geographic locations. Here, we explore the relationship of climatic characteristics such as mean annual precipitation and interannual variability and mean quarterly temperature to ecosystem traits including species diversity, cover, aboveground primary production, and decomposition using observed communities across a global network of 45 grassland sites. Our preliminary results suggest that although all of the sites are grass-dominated ecosystems, the variables controlling major ecosystem functions, such as aboveground primary production and decomposition, vary significantly among sites. This global scale study demonstrates the importance of within-biome variation in response to abiotic conditions, highlights the need for incorporation of this type of variability into existing dynamic global vegetation models, and provides validation data for such models. This analysis provides novel insights into the diversity of ways in which species richness as well as carbon sequestration and release in grasslands may be modified under a changing climate.

Abstract: A primary goal in community ecology has been to determine the constraints on the number of species that can coexist at a given site. The abiotic environment directly affects species richness by defining a fundamental niche. The abiotic environment also has indirect effects on species richness that are mediated through system productivity as reflected in species realized niches (e.g., the diversity-productivity relationship). Here, we use structural equations models to examine the relative importance of direct and indirect effects of the abiotic environment using a network of XX sites from grasslands around the world.

Abstract: Species invasions are one of the most serious threats to global diversity and impart hundreds of billions of dollars of costs each year. The key effects of exotic invasion on native communities are to reduce the abundance and richness of the native community and to erode its spatial structure. Exotic impact is most naturally defined as the dominance of exotic species in a community. Nevertheless, most regional-scale studies have focused on analyzing the number rather than dominance of exotic species, because of the difficulty of combining data on abundance and spatial structure across different studies. Here we examine the relative effects of exotic richness and abundance on the richness, abundance, beta diversity, and spatial covariance across a global network XX grasslands sampled using standard methods.